This invention relates to grinding worms for the continous generating grinding of gear wheels.
Thanks to the improvements in the properties of the abrasive grain, the bond and structure, the hard fine machining of gears with single-start or multi-start grinding worms has recently undergone a high increase in production rate, and due to its high quality and increased economic efficiency gained steadily in significance in gear manufacture. In combination with the improved dynamic properties of tooth flank grinding machines as brought about by NC-techniques, systematically developed specific profiling and grinding technologies provide for high accuracy, flexibility in the shaping of even intricately modified tooth flanks, and high degree exploitation of the tools. In particular the high grinding speeds nowadays possible not only with galvanic but also with re-dressable bonds have--by the resulting increase in metal removal rate--contributed much to the increase in grinding performance and economy of this process. The centrifugal forces occurring at high grinding speeds become so great, however, that profitable grinding worms with volumetric grinding wheel structure in vitrified, synthetic resin or other volumetric bond grow outwards due to the low bond stiffness and inhomogenities in the grinding body, and on account of the inevitable out-of-balance change their shape in an uncontrollable manner, whereby due to the incurred radial and axial runout of the working surface of the grinding worm--the flanks of the worm thread--the flank accuracy of the workpiece is impaired. This effect is further increased by the fact that there is usually play between the bore of the grinding worm and the diameter of the grinding worm flange, and the grinding worm is only held by the friction of the axial clamping between the shoulder and flange cover of the grinding worm flange, in which case due to out-of-balance in the grinding body under the influence of increasing centrifugal forces a radial displacement of the grinding body can occur. It is easy to see that by re-balancing on the machine, the excitement of vibrations in the grinding spindle can be minimized, but that geometrical alterations in the grinding worm due to variable peripheral speeds cannot be compensated.
The influence on the grinding accuracy due to displacements and deformations of the grinding body caused by centrifugal force is suppressed so long as profiling can be performed at the same grinding worm speed as later grinding, so that the shape of the grinding worm thread produced when profiling is maintained when grinding. It becomes a problem, however, when the grinding speed exceeds the profiling speed, the latter being limited by the motion sequence between grinding worm and profiling tool necessary for the profiling action, or by the dynamics of the NC-axes drives producing it. This happens especially with grinding worms of very hard abrasive, e.g. CBN or diamond, and with grinding worms of small outside diameter. The problem is intensified when with single-start worms the ends of the grinding worm thread which produce the out-of-balance do not lie on the same envelopment line of the grinding worm, so that their dynamic state of balance alters with the decreasing of the grinding worm diameter caused by profiling. Modern tooth flank grinding machines are in fact provided with a balancing device on the grinding spindle. But this can only improve the balance of the set-up flange with grinding worm. Due to the differing densities of the grinding worm and the set-up flange, the radial runout of the two centres of gravity causing the out-of-balance will differ, and hence also the radial runout of their outside diameters, so that the balanced state achieved on the machine is not identical to an optimum concentricity of the grinding worm thread. Moreover the grinding machine only allows compensation of the static out-of-balance, but not the dynamic, as the balancing lugs are arranged in one plane.
One of the possibilities for avoiding this problem lies, for example, in the use of grinding worms with the galvanic bonding of the abrasive grains on a worm-shaped steel body. This is the favoured method for grinding with CBN (Cubic Boron Nitride), which is extremely wear resistant. Such grinding worms are located without play on the grinding spindle, and thanks to the material strength the thread geometry of the grinding worm remains unaltered even at high grinding speeds. They are, however, not redressable. The geometry of the grinding worm threads produced when manufacturing the grinding worm cannot therefore be modified on the grinding machine if it were desirable, and stays the same throughout the life of the coating. Another disadvantage is that the grinding behaviour of such grinding worms alters in the course of their useful life, and that to achieve a high metal removal rate on the one hand and a low workpiece surface roughness on the other, grinding worms with different grain size coatings must be employed. Furthermore specific equipment and experience is required for the galvanic coating of the grinding worms, so that after every durability period they must be recoated with fresh abrasive grains by an external tool manufacturer. For this reason the use of such grinding worms is restricted to large series production, where the high flexibility with respect to flank shape, as offered by the profiling of redressable grinding worms on the machine, can be dispensed with.
In U.S. Pat. No. 5,954,568, a process for solving the problem is proposed, in which by the use of a dressing worm coated galvanically with diamond grains, even grinding worms for high grinding speeds can be profiled at grinding speed. One of the disadvantages of this method lies in the great number of diamond dressing worms required to embrace the workpiece module range of the machine. The costs involved and the technical command over the dynamically difficult dressing process have as yet prevented this solution from becoming popular.
U.S. patent application Ser. No. 09/476 994 suggests a process in which the geometrical errors in the grinding worm deriving from the deformation caused by the difference between the profiling and grinding speeds are determined by grinding a test specimen or by the measurement of the grinding worm threads at grinding speed via a sensor, and allowing for these errors in the form of corrections in the subsequent profiling operation. This method is a sure way of solving the problem. But it is complicated, and to maintain a high workpiece accuracy the error determination must be repeated frequently as the diameter of the grinding worm threads diminishes, which demands repeated stoppage of the production process with corresponding time loss.
Another way of reducing the detrimental influence of the speed difference between profiling and grinding is described in DE 4403 236 A1 (U.S. patent application Ser. No. 204 595). In order to minimize the change in the dynamic state of balance of singlestart grinding worms occurring with diminishing diameter of the grinding worm thread, the proposal is made to dimension the grinding wheel facewidth in the worm thread zone such that the entry and exit ends of the thread lie on the same envelopment line of the grinding worm periphery, equivalent to an integer number of thread windings. This measure only solves part of the problem, however; for the influence of the bore clearance and of the inhomogenities in the grinding body are still present.